Method and apparatus for processing downlink digital voice data

ABSTRACT

A method and apparatus for preprocessing digital voice data enroute to or from a Telephone&#39;s CODEC. The apparatus supports a CODEC directly connected to an internal bus and using a separate CODEC clock and sync signal to control transfers between a telephony link/internal bus interface and the CODEC. According to an embodiment of the present invention, an auxiliary processing device (connected to the internal bus side of the phone) is provided with means to process the digital voice information before sending it uplink or to the CODEC. This is accomplished without changing the position of the incoming voice field.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to telephony communications and related telephonebus protocols.

Related Art

A digital telephone may conventionally include at least two types ofports: a telephone line interface for connection with a telephonenetwork, and a data communications interface for communications with acomputer. For example, the ROLMphone (R) 244PC includes a telephony portfor providing connection to a telephone network switch, such as aprivate branch exchange (PBX), and two data communications ports forproviding connection to a computer.

In conventional telephones, the telephony communications link may beprovided using a variety of protocols. For example, in the 244PC theconnection to the network switch is accomplished by way of ROLMlinkprotocol. In many other conventional telephones, the connection to theswitch is established by using an industry standard protocol such asISDN.

A ROLMlink connection carries four types of information: digital datacommunications control, digital data communications, digital voice andgeneral control. Digital data communications control refers toinformation concerned solely with controlling digital data connectionsand status. General control is concerned with control of the telephoneas a whole. The information is organized into "fields" (groups of bitsassociated with one of the four information types). The fields areconcatenated into a ROLMlink "frame" which contains one of each of thefields. Transfers on the link from the switch to the phone are said tobe "downlink" while transfers from the phone to the switch are said tobe "uplink".

Once the information arrives at the telephone, it is distributed to thetelephone's various components by way of an internal bus.Conventionally, the telephony link is connected to the internal bus byway of a link interface, which translates the telephony information intothe proper format and timing in accordance with the internal busstandard.

The internal bus may be one of a number of types. A conventionalstandard for such internal busses is General Circuit Interface (GCI).The GGCI standard, describes, among other things, the structure of dataframes and the timing of certain signals.

In prior art digital telephones, digitally encoded audio informationarriving from the telephony link is sent from the link interface to acoder/decoder (CODEC). The CODEC is a digital-to-analog andanalog-to-digital converter, specialized for telephone applications. TheCODEC converts digital audio information into analog form to drive aspeaker and/or handset, and converts the analog information from amicrophone or handset to digital information destined for the telephonenetwork switch.

Typically, the CODEC uses a timing pulse, generated by the linkinterface, to identify the particular time slot designated to hold theincoming or outgoing digital audio information within each data frame.Since the audio information is typically located in a fixed time slotwithin every frame, the link interface is conventionally set up once, atinitialization time, to assert the timing pulse in coincidence with thisfixed time slot.

SUMMARY OF THE INVENTION

For various reasons, it is sometimes desirable to intercept andpreprocess the digital audio information enroute between the CODEC andthe telephone link interface. Thus, in accordance with the presentinvention, there is provided a method and apparatus for preprocessinguplink or downlink digital voice data.

The apparatus supports a CODEC directly connected to the telephone'sinternal bus, using a separate CODEC clock and sync signal to controltransfers between the link interface and the CODEC. According to anembodiment of the present invention, another downlink device (connectedto the internal bus side of the phone) is provided with means to processthe digital voice information within a data frame enroute between theCODEC and the link interface. The processed information is placed in anauxiliary field within the same or a subsequent data frame and the CODECclock is shifted accordingly, thus eliminating the need to change theposition of the switch originated or destined voice field on theinternal bus side of the link interface. Advantageously, this enablesinformation processed by an auxiliary device to be handled by thetelephone without the need for a programmable CODEC.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will be better understood by reference to thedrawing, wherein:

FIG. 1 is a diagram of a digital telephone according to an embodiment ofthe present invention;

FIG. 2 is a more detailed diagram of the link interface of FIG. 1;

FIG. 3 is a timing diagram illustrating relationship between the dataframe voice fields and the CODEC clock signals according to anembodiment of the present invention;

FIG. 4 is a flow chart of the operation of the telephone of FIG. 1according to an embodiment of the present invention;

FIG. 5 is a more detailed illustration of the GCI interface of FIG. 2;

FIG. 6 is a more detailed illustration of the Digital DataCommunications Register of FIG. 5; and,

FIG. 7 is a timing diagram of the control signals used in the DigitalData Communications Register of FIG. 6.

Like numbered reference numerals appearing in more than one figurerepresent like elements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An improved telephone, according to an embodiment of the presentinvention is illustrated schematically in FIG. 1. The telephone includesa link interface 102 which translates ROLMlink protocol to thetelephone's internal bus protocol and which handles transfers oftelephone control information between a microprocessor 104 and theROLMlink line.

The microprocessor 104, is connected to the link interface 102. Themicroprocessor 104 controls all of the telephone functions, and is thesource and destination for all communications with a PBX switch. Alsoconnected to the link interface and the microprocessor is a keypad andassociated control logic 106. The keypad/control logic 106 includes thetelephone keys, lights and the associated control circuitry.

A coder/decoder (CODEC) 108 is connected to receive data output from thelink interface 102. The CODEC 108 converts digital audio informationinto analog form to drive a speaker 110 and/or handset 112, and convertsthe analog information from a microphone 114 and/or the handset 112 todigital information destined for the network switch. The CODEC 108 isalso connected to receive two clock signals from the link interface. Thefirst of these signals, the CODEC sync (CSYNC) 116, tells the CODEC whento read a field in an data bus frame. The second, the CODEC clock (CCLK)118, is a bit clock which runs at the bit transmission rate of the GCIbus.

A digital LCD display 120 is also connected to the microprocessor 104.The display 120 is used to display data such as the number dialed,stored telephone numbers and other data provided by way of themicroprocessor 104.

A data communications controller (DCC) 122, controlled by themicroprocessor 104, provides the telephone with two RS-232 datacommunications interfaces. The DCC 122 is connected to the linkinterface 102, the microprocessor 104 and the CODEC 108. Commandsreceived by way of the RS-232 data communications ports are sent to themicroprocessor by way of the data path. Data from an optionallyconnected RS-232 device flows through the telephone and ROLMlink to theswitch, from which it flows to another line or device.

The data bus 124 is based on the conventional GCI specification forterminals (GCI terminal mode). The data bus uses the GCI electricalspecifications, timing and signal lines as well as the overall GCI framestructure. In the preferred embodiment, however, the specific contentsof the frame fields as well as their individual uses and protocols neednot adhere to the GCI specification. For purposes of this document, thedata bus frames will be referred to as GCI frames even though they neednot fully comply with GCI field definition and protocol specifications.Each frame includes a field for the Data Communications and Data Control(DCDC), data communications, ROLMlink voice and auxiliary voice fields.

The control bus 126 is the path for phone control information which issourced by the microprocessor. An audio bus 128 carries analog audioinformation and provides an analog path to and from the CODEC 108.

An auxiliary processing apparatus 130 is connected to the link interface102, the microprocessor 104 and may also be connected to the CODEC 108.The auxiliary processing apparatus can be of a "plug in" type which isinstallable by the user. Such auxiliary devices may be used for a numberof purposes. However, as related to the present invention, the auxiliaryprocessing device is of a type which preprocesses digitally encodedaudio information in a GCI frame destined for the CODEC. Themicroprocessor 104 detects the presence or absence of the auxiliaryprocessing device 130 and receives related control parameters, describedlater, by way of a handshake data exchange between the two (104, 130) attelephone initialization time.

A more detailed view of the link interface 102 is shown in FIG. 2. TheROLMlink interface 202 receives downlink data (LR), in ROLMlinkprotocol, from the switch side of the telephone, and transfers the datato and from an internal bus 204 which, in turn, connects to a GCIinterface 206. The ROLMlink interface also takes the data from theinternal bus 204 and converts it into positive and negative uplinksignals (LD, LD*) for transmission on the ROLMlink line. Controlinformation is passed directly to a microcontroller interface 208.

The GCI interface 206 transfers information between the data bus 124 andthe internal bus 204. The GCI interface 206 also generates the CSYNC andCCLK signals for the CODEC 108. The Data Downstream (DD) signal is thedownlink GCI data generated by the GCI interface. Incoming or uplink GCIdata is carried by the Data Upstream (DU) signal. The FSC signal is aframe sync which is indicative of the start of each data frame. The GCIData Clock (DCL) is a clocking signal which runs at twice the GCI busbit transmission rate in sync with bit transfers on the GCI bus.

A base interface 210 generates local phone control signals under controlof the microprocessor 104. These signals include control informationsuch as the ring tone output, the key tone output, speakerpower-enable/voice-active.

The microcontroller interface 208 handles communications between theROLMlink interface 202 and the microprocessor 104. Incoming signalsinclude those such as the Serial Peripheral Interface (SPI) data clock,SPI slave select, and SPI data in. The outgoing signals include, SPIdata out, the processor interrupt output, external processor reset, andthe processor clock.

A more detailed diagram of the GCI interface 206 is shown in FIG. 5.Control and timing circuitry 502 is provided to issue the timing signalsFSC, DCL, CSYNC and CCLK, and a number of other internal timing signals(on bus 503) which will be described in more detail later. A ROLMlinkvoice register 504 holds digital voice data being transferred betweenthe ROLMlink and data bus 124 sides of the link interface 102. A digitaldata communications register 506 holds data communications and datacontrol information (DCDC) and digital data communications information(DATA). A 3:1 multiplexer (MUX) 508 is connected to the ROLMlink voiceregister 504, the Digital Data Communications Register 506 (via line510) and a "logical one" (high) level voltage source. The Control andtiming circuitry is connected to the MUX 508 select inputs and controlsthe MUX 508 so as select which of the registers 504, 506 or the logical"1" are output to the GCI signal line DD.

The communications registers 506 can be embodied in a number of ways.For example, DCDC and data flowing downlink can be collected and held ina first "downlink register" and uplink information can be collected andheld in a second "uplink" register. In this "two register" embodiment,the DCDC and data fields from the ROLMlink are collected in the"downlink" register and then transferred to the data bus 124 in theappropriate time slot within a GCI frame. At the same time that the"downlink" register is transferring data to the data bus 124, the"uplink" register collects uplink data from the data bus 124. At theappropriate time slot within a ROLMlink frame, the "uplink" registertransfers it's data to the uplink ROLMlink data flow. While thistransfer is occurring, the "downlink" register is collecting the nextbatch of downlink data.

An improved embodiment of the communications register is illustrated inFIG. 6. In the embodiment of FIG. 6, both downlink and uplink data arecollected and transferred by one 16 bit serial shift register 602. Theclocking of the serial shift register 602, and thus the timing of datacollection, is selected by a 3:1 MUX 604. The three inputs to the MUX604 are ground (no clocking), the ROLMlink bit clock and the GCI receiveclock. The selection of the clocking signals (no clock, ROLMlink bitclock, GCI receive clock) is controlled by two select signals (SELECT1,SELECT2), generated by the timing and control circuitry 502 on bus 503.

Both the ROLMlink bit clock and the GCI receive clock are also generatedby the timing and control circuitry 502 on bus 503. The ROLMlink bitclock is a bit rate clock for the ROLMlink frames. The GCI receive clockis a bit rate receive clock for the GCI frames. The GCI frames are threetimes as long as the ROLMlink frames (96 bits as compared with 32 bits)and are transmitted at a transmission rate three times that of theROLMlink frames. Thus, the GCI receive clock runs at three times therate of the ROLMlink bit clock.

The input of the shift register 602 is connected to the output of afirst 2:1 MUX 606 which is, in turn connected to receive the ROLMlinkdata (from bus 204) and DU signals. The first 2:1 MUX 606 selects thedata to be collected under control of the SELECT2 signal. A second 2:1MUX 608, connected to the output of the shift register 602, selects thefield in which to inject collected ROLMlink data into uplink ROLMlinkdata frames. The second 2:1 MUX is also controlled by the SELECT2signal.

The timing of the control and clocking signals related to FIG. 6 areillustrated in FIG. 7. The CLK waveform 702 represents the output of the3:1 MUX 604. The 3:1 MUX 604 selects it's ground input (input A) whenSELECT1 is low. When SELECT1 is high, the MUX 604 selects input A whenSELECT2 is low and input B when SELECT2 is high. Similarly, the 2:1MUXes 606, 608 will select input A when SELECT2 is low and input B whenSELECT2 is high.

The 3:1 MUX 508 of FIG. 5 operates under control of the SELECT2 andSELECT3 signals. As with SELECT1 and SELECT2, the SELECT3 signal isgenerated by the timing and control circuitry 502 on bus 503. WhenSELECT3 is low, MUX 508 will select its "high" (logical "1") input. WhenSELECT3 is high, MUX 508 will select the output of register 506 (the GCIframe DCDC and DATA fields) when SELECT2 is high and the output ofregister 504 (the ROLMlink voice field) when SELECT2 is low.

During time period 704, downlink ROLMlink data (RDATA) is collected inthe register 602. At the same time, uplink ROLMlink data (TDATA),previously collected from the data bus 124 (as DU) is transferred on theoutput of the second 2:1 MUX 608 to the uplink ROLMlink line. Later,during time period 706, when the appropriate GCI frame fields arepresent, the uplink GCI frame data is collected in the register 602,while the previously collected ROLMlink data is transferred (as DD) to adownlink bound GCI frame via MUX 508 (FIG. 5).

It should be understood that the logic of FIG. 6 is not limited to frametypes having the same field position definitions. By varying the timingof the SELECT1, SELECT2 and SELECT3 control signals, one or more fieldsfrom a position within a first frame of a first format or type (e.g. aROLMlink frame) can be injected into a different field position within asecond frame of a second format or type (e.g. a GCI frame). For example,DCDC and DATA fields in positions one and two (as shown in the RDATA andTDATA frames of FIG. 7) can be injected into positions three and five ofa another frame.

A more detailed example of a GCI frame is illustrated in FIG. 3. As withthe incoming ROLMlink frame, the GCI frame includes a number of fields.In accordance with an embodiment of the present invention, the GCIframes include a DCDC field 302, a DATA field 304, a ROLMlink voicefield 306 and an Auxiliary Voice field 308. The DCDC field containscontrol information related solely to the ROLMlink DATA field. The DATAfield 304 contains the raw, digital data communications information, theROLMlink voice field 306 contains the digitally encoded voiceinformation taken from the ROLMlink line. The Auxiliary Voice field isput to special use. Specifically, this field contains processed digitalvoice information from or to a downlink auxiliary device.

The operation of a telephone auxiliary device according to an embodimentof the present invention will be better understood by reference to FIG.4. When the telephone is initialized, in step 402 the presence orabsence of an AUXILIARY device is detected by the microprocessor by wayof signals of the control bus. If the AUXILIARY device is not present,no request is received to move CSYNC in step 404. Thus, in step 406, themicroprocessor programs the link interface to assert CSYNC incoincidence with the timeslot for the ROLMlink voice field (asillustrated by waveform 312). If the AUXILIARY device is present, atinitialization time it sends control data, to the microprocessor,indicative of the time slot in which CSYNC is to be asserted. If thissignal is detected in step 402, in step 408 the microprocessor, in turn,programs the link interface to assert CSYNC at the commencement of thetime slot requested by the AUXILIARY device. Thus, in step 412, theauxiliary device, if present, intercepts the voice data received fromthe appropriate field in a GCI frame and retransmits the processed dataduring the programmed CSYNC time slot determined by step 408. If noauxiliary device is present, the CSYNC will be determined by step 406.

Whenever a telephony frame arrives from the telephony link, the linkinterface converts it to a GCI frame. Where the frame is transferred aspart of a "voice" connection (as indicated by a control message withinthe general control field) the link interface sends the GCI frame to theauxiliary device. The GCI frame and the CSYNC signal are also sent tothe CODEC. In contrast to the conventional link interfaces, however, thelink interface of the present invention does not assert the CSYNC signaluntil the time slot requested by the AUXILIARY device. Thus, theauxiliary device has the opportunity to process the digital voice data,place the data in a later fixed time slot (the auxiliary voice field)308. As described above, the AUXILIARY device communicates with themicroprocessor at initialization time so as to cause CSYNC to beasserted only during the auxiliary voice field time slot 308 (asillustrated by waveform 314).

In step 410, the Auxiliary device receives the GCI frame. The AUXILIARYdevice uses the Data Clock (DCL) and Frame Sync (FSC) signal (waveform316), generated by the link interface, to determine which time slot isoccurring. The FSC signal indicates when the link interface hascommenced transmission of a frame on the data bus 124. The data clocksare generated by the link interface at twice the bit transmission rateon the data bus 124. At the commencement of the FSC signal, theAUXILIARY device begins to count Data Clocks to determine the currentbit position. From the perspective of the link interface, oncetransmission of the frame has been commenced, the link interface countsthe number of data clocks (DCL). Since the AUXILIARY device knows thatthe ROLMlink voice field will always be in the same time slot withineach frame (and thus starting at the same bit position), it counts thedata clocks to determine when to start reading the voice data. Forexample, where the ROLMlink voice data is in the GCI ICI position, theAUXILIARY device will start capturing data on the fifth field of data(i.e. the 66th bit clock given 8 bit data fields).

After the AUXILIARY device has completed processing of the digital voicedata, in step 412 it inserts the processed data into a later time slotin the same or a subsequent frame. This time slot is referred to as theAUXILIARY voice field 308.

As previously described, the AUXILIARY device signals the linkinterface, at initialization time, to raise the CSYNC signal such thatthe CODEC will read the AUXILIARY voice field rather the ROLMlink voicefield. Thus, the processed data is made available to the CODEC in a timeslot (within the same or a subsequent frame) determined by the AUXILIARYdevice.

As an alternative embodiment, the AUXILIARY device can program thetiming of the CSYNC signal on the fly. In other words the AUXILIARYdevice can process the data, determine which time slot to place theprocessed data within (based on actual or projected processingrequirements) and then program the link interface to generate CSYNCaccordingly.

The AUXILIARY field selected for the processed data does not need todirectly follow the ROLMlink voice frame (IC1). Further, the AUXILIARYdevice can place the processed data into an AUXILIARY field within asubsequent frame and program the link interface to generate CSYNCaccordingly. For example, where several frames (e.g. 3) of ROLMlinkvoice data need to be processed in order to obtain a result, the resultcan be placed in several fields of the last frame needing to be mergedand several CSYNC signals (e.g. 3) generated accordingly.

The forgoing principles apply to both uplink and downlink data. In thedata bus 124, frames of uplink data are sent on one carrier (bus line)in synchronism with frames of downlink data sent on a second carrier.Thus, timing of CSYNC can be programmed such that the CODEC will placeunprocessed uplink data into the auxiliary voice field. This uplink datacan then be processed by the AUXILIARY device and placed in the ROLMlinkvoice field of the same or a subsequent frame for transmission on thetelephony line.

As an alternative to moving the CSYNC signal, the link interface can beprogrammed to change the position of the ROLMlink voice field when anauxiliary device is present. In this embodiment, the microprocessor 104detects the presence of the AUXILIARY device 130 via a handshake attelephone initialization time. In response to detection of the AUXILIARYdevice 130, the microprocessor programs the link interface 102 to placethe ROLMlink voice field (from incoming ROLMlink frames) into the IC2position of downlink GCI frames and to expect the ROLMlink voice fieldin the IC2 position of uplink GCI frames. The CSYNC signal remainsprogrammed for the IC1 time slot. Once this has been accomplished, theAUXILIARY device handles downlink frames by processing the data from theIC2 position and placing the results into the IC1 position. For uplinkframes, the AUXILIARY device 130 processes data from IC1 and places theresults into IC2.

The forgoing principles can also be applied to data other than digitalvoice data. For example, by manipulating the position of the CSYNCsignal in telephones and other devices having a video CODEC, theAUXILIARY device can process downlink or uplink video data from a firstfield of one frame and then put the processed data into a subsequentfield of the same or a later frame.

Now that the invention has been described by way of the preferredembodiment, various enhancements and improvements which do not departfrom the scope and spirit of the invention will become apparent to thoseof skill in the art. The preferred embodiment is described as an exampleand not as a limitation. The scope of the invention is defined by theappended claims.

We claim:
 1. A digital telephone, comprising:a telephony link interface,the telephony link interface comprising means for generating a dataframe including a first data field for holding incoming digitalinformation and a second data field for holding auxiliary digitalinformation; auxiliary data processing means, coupled to the telephonylink interface, for processing the incoming digital information withinthe first data field, said auxiliary data processing means includingmeans for placing results of the processing into the second data field;detection means, for detecting presence of the auxiliary data processingmeans in said digital telephone; control means, coupled to the detectionmeans, for normally generating a control signal indicative of a positionof the first data field and, when the auxiliary data processing means isdetected, for generating the control signal so as to be indicative ofthe position of the second data field; and, a coder/decoder coupled tothe telephony link interface, the auxiliary data processing means andthe control means, the coder/decoder including: means, responsive to thecontrol signal for selecting a data field from the data frame and forconverting contents of the data field into an analog signal.
 2. Thedigital telephone of claim 1 wherein the control means comprises logicfor generating the control signal at a time slot indicated by theauxiliary data processing means as holding the second data field.
 3. Thedigital telephone of claim 1 wherein, for every data frame generated bythe telephony link interface, the time slot is at a fixed positionrelative to the first data field.
 4. The digital telephone of claim 1wherein said incoming digital information is voice information andwherein said analog signal is an audio signal.
 5. The digital telephoneof claim 1 wherein said incoming digital information is videoinformation and wherein said analog signal is a video signal.
 6. Thedigital telephone of claim 1 wherein the control means comprises meansfor detecting the presence and absence of the auxiliary data processingmeans and means for generating the control signal in coincidence withthe first data field when the absence of the auxiliary data processingdevice is detected and in coincidence with the second data field whenthe presence of the auxiliary data processing device is detected.
 7. Adigital telephone, comprising:a telephony link interface, the telephonylink interface comprising means for generating a data frame including afirst data field for holding incoming audio information and a seconddata field for holding processed audio information; a coder/decodercoupled to the telephony link interface, the coder/decoder including:means for selecting a data field from the data frame and for convertingcontents of the data field into an analog audio signal; and, auxiliarydata processing means, coupled to the coder/decoder and the telephonylink interface, for processing the incoming audio information within thefirst data field, the auxiliary data processing means including meansfor placing results of the processing into the second data field andmeans for causing the coder/decoder to select the second data field andnot the first data field.
 8. A plug-in auxiliary device for use in adigital telephone of a type in which telephony information is processedas a plurality of frames, each of the frames including a first datafield for holding incoming digital audio information, the digitaltelephone further being of the type which includes a coder decoder forconverting a selected field within said frame to an analog audio signal,the coder/decoder normally selecting the first field for conversion tothe analog audio signal, said auxiliary device comprising:means forprocessing the incoming audio information within the first data field,means for placing results of the processing into a second data fieldwithin one of said frames, means for causing said coder/decoder toselect said second data field and not said first data field forconversion to the analog audio signal.
 9. A method of operating adigital telephone, comprising the steps of:detecting presence of anauxiliary data processing apparatus coupled to said digital telephone;receiving a stream of telephony data frames, each comprising a firstdata field for holding incoming digital information; processing, by theauxiliary data processing apparatus, the incoming digital informationwithin the first data field of a first data frame; placing results ofthe processing into a second data field of a subsequent data frame, saidsecond data field being at a different time slot, in each of aid dataframes, than said first data field; responsive to detection of thepresence of the auxiliary data processing apparatus, generating acontrol signal indicative of a position of said second data field withinsaid subsequent data frame; and, responsive to said control signal,selecting the second data field from said subsequent data frame andconverting contents of said second data field into an analog signal. 10.The method of claim 9 wherein the processing is performed by anauxiliary data processing apparatus and wherein the control signal isgenerated at a time slot indicated by the auxiliary data processingapparatus as holding the second data field.
 11. The method of claim 9wherein, for every data frame generated by the telephony link interface,the time slot is at a fixed position relative to a time slot of thefirst data field.
 12. The method of claim 9 wherein said incomingdigital information is voice information and wherein said analog signalis an audio signal.
 13. The method of claim 9 wherein said incomingdigital information is video information and wherein said analog signalis a video signal.
 14. A method of operating a digital telephone,comprising the steps of:detecting presence of an auxiliary dataprocessing apparatus coupled to said digital telephone; receiving astream of telephony data frames, each comprising a first data field forholding incoming digital information; processing, by the auxiliaryprocessing apparatus, the incoming digital information within the firstdata field of a first data frame; placing results of the processing intoa second data field of the first data frame, said second data fieldbeing at a different time slot, in each of said data frames, than saidfirst data field; responsive to detection of the presence of theauxiliary processing apparatus, generating a control signal indicativeof a position of the second data field within the first data frame; and,responsive to said control signal, selecting the second data field fromthe first data frame and converting contents of the second data fieldinto an analog signal.
 15. The method of claim 14 wherein the processingis performed by an auxiliary data processing apparatus and wherein thecontrol signal is generated at a time slot indicated by the auxiliarydata processing apparatus as holding the second data field.
 16. Themethod of claim 14 wherein, for every data generated by the telephonylink interface, the time slot is at a fixed position relative to a timeslot of the first data field.
 17. The method of claim 14 wherein saidincoming digital information is voice information and wherein saidanalog signal is an audio signal.
 18. The method of claim 14 whereinsaid incoming digital information is video information and wherein saidanalog signal is a video signal.
 19. In a digital telephone forreceiving a plug-in auxiliary device, an internal bus which carries dataframes having a plurality of fields; and, a coder/decoder coupled to theinternal bus, the improvement comprising:control means for detectingpresence and absence of an auxiliary data processing device, the controlmeans including position indication means for generating a controlsignal indicative of a position of a first one of the fields when theabsence of the auxiliary data processing device is detected and forgenerating a signal indicative of a second one of the fields when thepresence of the auxiliary data processing device is detected; and, acoder/decoder coupled to the telephony link interface and the controlmeans, the coder/decoder including means for selecting a field from thedata frames in accordance with the control signal and for convertingcontents of the field into an analog signal.
 20. A method of operating adigital telephone, comprising the steps of:detecting presence andabsence of an auxiliary data processing apparatus in said digitaltelephone; receiving a stream of telephony data frames, each comprisinga first data field for holding incoming digital information and a seconddata field for holding auxiliary digital information; when the auxiliarydata processing device is present in said digital telephone, processingthe incoming digital information within the first data field and placingresults of the processing into a second data field; generating a controlsignal in coincidence with the first data field when the absence of theauxiliary data processing device is detected and in coincidence with thesecond data field when the presence of the auxiliary data processingdevice is detected; selecting the a data field from the data frames inaccordance with the control signal and converting contents of the datafield into an analog signal.